docs: add ZvenoAI OpenAI-compatible config

[nikitaxru]

Description

Adds ZvenoAI to the OpenAI-compatible providers list and includes a minimal config.yaml example using the production endpoint, a current tool-capable model, and Continue's secret syntax. The guide links to the ZvenoAI model catalog for other model IDs.

Refs #13156.

Checklist

• I have read the contributing guide.
• The relevant documentation has been updated.
• Tests are not applicable to this documentation-only change.

Screen recording or screenshot

Not applicable for a configuration example.

Tests

• npx --yes prettier@3.3.3 --check docs/customize/model-providers/top-level/openai.mdx
• Compiled the MDX with @mdx-js/mdx and parsed all five YAML examples with js-yaml.
• git diff --check
• Confirmed that openai/gpt-oss-20b:free lists tools and tool_choice in the production /v1/models response.

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[nikitaxru]

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